So-called liquid crystal displays (LC displays) are largely dominant today on the flat video screen market. LC displays are distinguished by low-cost production, low electrical power consumption, low weight, and small space requirement. In addition, or course, LC displays also have drawbacks, which consist foremost of the fact that these video screens are not self-illuminating and therefore can be read or recognized only under suitable ambient light conditions.
Displays based on organic light-emitting diodes, OLEDs (Organic Light Emitting Diodes), have acquired a name for themselves since 1987. They consist in principle of electroluminescent organic films that are positioned between two electrodes. When an electrical potential is applied to the electrodes, light is emitted because of the recombinations between electrons and “holes”, which are injected into the organic films. Because of the self-emissivity of OLEDs, with them there is no need for the backlighting that is often necessary with LC displays. The space requirement and electrical power consumption of OLEDs is thereby reduced considerably. Switching times are in the range of one microsecond and are only slightly temperature-dependent, which makes possible their use for video applications. The reading angle is nearly 180°. Polarizing films such as those necessary with LC displays are usually not needed, so that greater brightness of the display elements can be produced. Another advantage is the ability to use flexible and non-planar substrates.
A transparent electrode (anode) of indium-tin oxide (ITO) is used for OLEDs in most cases. ITO is usually applied to the whole surface of a substrate, which is generally transparent, and is then structured by a photolithographic process followed by etching with HBr, which produces the desired form of the electrodes. Electrodes are structured in the form of parallel electrode strips as a rule for use as a passive matrix display.
For special applications, for example for full-color displays, additional layers such as color filters can be located between the substrate and the ITO electrode strips. The functional organic films are then applied to the structured anode. In case of films that consist of low molecular weight systems, for example hydroxyquinoline-aluminum(III) salts, this is usually done by a thermal vaporization process under vacuum. When electroluminescent polymers are used, the functional films can be applied by overall coating processes from solution, for example by blade coating, spin coating, or special printing methods, for example screen printing or ink jet printing. The publication EP 0 892 028 A 2 discloses how to apply functional polymers in the windows of a window layer that define the image points, by means of a contactless ink jet printing process. Multilayered functional films can also be made by this contactless printing process. The publication WO 99/07189 discloses a number of standard printing processes, for example web printing, offset printing, and screen printing processes, for applying electroluminescent polymers.
The second electrode, the cathode, can then typically be applied to the organic electroluminescent films by vapor deposition through a shadow mask. The size, shape, and separation of the cathode structures that can be produced are limited by the ability to manipulate these masks. To circumvent this problem for various applications, for example passive matrix displays, parallel rows of strip-shaped ridges with breakaway edges are constructed of photoresist on the first electrode strips, as disclosed, for example, in the publication EP 0 910 128 A2. Parallel cathode paths with defined width and spacing can be produced perpendicular to the anode strips by the shape and positions of these strip-shaped ridges, by vapor deposition of an overall metal coating and then removing it at the breakaway edges of the strip-shaped ridges, so that the cathode strips are produced. This method has the drawback that an additional process step is necessary, namely the structuring of the strip-shaped ridges.
It is known from the publications WO 99/39373 and WO 99/43031 how to produce the electrodes by a contactless ink jet printing process. However, this process is very slow and therefore very cost-intensive. Furthermore, it is very difficult to produce homogeneous electrode films with a definite thickness by the ink jet printing process.